1. Field of the Invention
The present invention relates generally to fuel assemblies for nuclear reactors and, more particularly, is concerned with a modified fuel assembly skeletal structure incorporating a combination of primary and secondary longitudinal structural support members tailored to meet mechanical strength requirements for the fuel assembly and designed for use where the moderator control function of the fuel assembly has been eliminated.
2. Description of the Prior Art
A typical pressurized water nuclear reactor (PWR) contains a large number of fuel assemblies in its core. The fuel enrichment of the fuel assemblies ordinarily varies from one group of assemblies to another depending upon their locations in the reactor core. In order to optimize the fuel burnup and smooth the radial neutron flux profile across the reactor core, historically a zoned enrichment pattern has been used.
Notwithstanding their variation in fuel enrichment, all of the fuel assemblies in the reactor core have the same construction. Basically, each fuel assembly is composed of a bottom nozzle, a top nozzle, an instrumentation tube and pluralities of guide thimbles, fuel rods and support grids. For instance, in one exemplary fuel assembly, the fuel rods are arranged in a square 17 by 17 array with 17 rod locations per side. Of the total possible 289 rod locations per assembly, 264 locations contain fuel rods. In addition to the single bottom nozzle, top nozzle and instrumentation tube, there are 24 guide thimbles and 8 support grids.
The structural skeleton of the fuel assembly is composed of the bottom and top nozzles and the plurality of guide thimbles which extend vertically between the bottom and top nozzles and rigidly interconnect them. In addition to their shared function with the bottom and top nozzles of providing the fuel assembly with a rigid skeleton, the guide thimbles provide channels through the fuel assembly which serve as a means for guiding the insertion of a rod control cluster (RCC) into the fuel assembly. In the standard 17 by 17 fuel assembly, the position of the 24 guide thimbles mirrors the position of the 24 control rods of the RCC.
A more recently-designed advanced PWR fuel assembly employs 16 large guide thimbles instead of the 24 small guide thimbles in the standard PWR fuel assembly. The large guide thimbles of the advanced PWR fuel assembly serve as potential locations not only for the RCC, but also for a gray rod cluster (GRC) or a water displacer rod cluster (WDRC). This latter component, the WDRC, performs the moderator control function in the advanced PWR and can be easily accommodated since only 8 of the 16 guide thimbles are dedicated to the RCC and only then when the fuel assembly is positioned in a "rodded" core location.
It is anticipated that some future uses of the advanced PWR fuel assembly will not include the moderator control function provided by the WDRC. If the design of this fuel assembly is not altered, then each of the fuel assemblies at control or gray rod locations would have 8 empty guide thimbles, while the remaining assemblies at "non-rodded" locations would each have all 16 guide thimbles empty. These empty guide thimbles represent potential "water-hole peaking" locations as well as a loss in fuel rod surface area. Preliminary calculations indicate that the peaking factors at the beginning of a cycle would be 5 to 10 percent greater. Also, loss of fuel rod surface area is especially severe in the advanced PWR fuel assembly where each of the 16 large guide thimbles displace the equivalent of four fuel rod positions. (In the standard PWR fuel assembly having the 24 small guide thimbles, each guide thimble displaces only one fuel rod position.)
Consequently, a need exists for a fresh approach to fuel assembly design which will anticipate future requirements and enhance adaptability of the fuel assembly to them without sacrificing its structural integrity and while avoiding or reducing some of the limitations and shortcomings inherent in the present advanced PWR fuel assembly construction described above.